Ohm's law calculator

Ohm's law relates voltage, current and resistance: V = R × I, giving I = V/R and R = V/I. Power is P = V × I.

Step-by-step example

A 12 V supply across a 4 Ω resistor:

• Current: I = 12 ÷ 4 = 3 A
• Power: P = 12 × 3 = 36 W

Power is where components fail

Ohm's law tells you the current; the power equation tells you whether the part survives. Substituting gives two more useful forms: P = I² × R and P = V² ÷ R. Note the squares — doubling the current quadruples the heat dissipated, which is why a resistor rated for a quarter watt burns out well before the current looks alarming.

Series and parallel

In series, resistances add and the same current flows through all of them. In parallel, the reciprocals add, so total resistance is always lower than the smallest branch — and each added branch draws more total current. It is the reason overloading a circuit happens by plugging in more devices, not bigger ones.